The present invention relates to a developing system for developing an electrostatic latent image in an electrophotographic copying machine and, more particularly, to a bias voltage control system in the developing system.
A bias voltage developing system has been developed, wherein a bias voltage is applied between a photosensitive drum and a magnet brush developing device for ensuring a clean developing. The bias voltage developing system is very effective for obtaining a clean background even though residual electric charges are held on the photosensitive drum at the background section. However, a stable developing operation was not ensured even in the bias voltage developing system when subjected to an extraordinarily high temperature, humidity condition or an extraordinarily low temperature, or humidity condition.
Under the high humidity condition, leakage current problems will occur in the corona charging device and in the photosensitive drum surface. And, under the low humidity condition, the corona charging current will increase and the resistance value of the photosensitive material will increase. These factors will influence the level of the electrostatic latent image formed on the photosensitive drum. Therefore, if the bias voltage has a fixed value, clean developing is not ensured when the humidity varies.
Accordingly, an object of the present invention is to provide a novel developing system for ensuring clean developing in an electrophotographic copying machine.
Another object of the present invention is to provide a magnet brush developing system, which ensures clean developing without regard to variations of the humidity.
Still another object of the present invention is to provide a control system for varying a bias voltage applied between a photosensitive drum and a magnet brush developing device in response to variations of the humidity.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
To achieve the above objects, pursuant to an embodiment of the present invention, a humidity sensor is disposed near a photosensitive drum in an electrophotographic copying machine for detecting the humidity in the copying machine, especially, the humidity of a photosensitive material. A voltage control circuit is associated with the humidity sensor for varying a bias voltage applied between the photosensitive drum and a developing device in accordance with an output signal derived from the humidity sensor.
In a preferred form, when the humidity sensor indicates that the humidity is above a preselected value, for example, 75%, the bias voltage is selected at a higher value, for example, 120 V. If the humidity sensor indicates that the humidity is below the preselected value, the bias voltage is selected at a lower value, for example, 60 V.